The present disclosure relates generally to LED lighting systems and LED control methods therefor.
LED lights have several advantages. For example, LEDs have been developed to have lifespan up to 50,000 hours, about 50 times as long as a 60-watt incandescent bulb. Furthermore, an LED requires minute amount of electricity, having luminous efficacy about 10 times higher than an incandescent bulb and 2 times higher than a florescent light. As power consumption and conversion efficiency are big concerns in the art, LED lights are expected to replace several kinds of lighting fixtures in the long run.
A LED is a current-driven device. As commonly known in the art, the brightness of a LED is substantially determined by its driving current, and the voltage drop across the LED when illuminating, commonly referred to as forward voltage, is about a constant. FIG. 1 shows LED lighting system 20 according to US patent application publication 20120217887, which is incorporated herein by reference in its entirety. LED lighting system 20 in FIG. 1 has LED string 14 with LEDs 15a, 15b and 15c connected in series. Bridge rectifier 12, connected to a branch circuit providing an alternative-current (AC) voltage VAC, generates input voltage VIN as an input power source to power LED string 14. Switch controllers Ca, Cb, and Cc control path switches Sa, Sb, and Sc, respectively, where each path switch is connected to a cathode of a LED. Mode decider 32 decides the operation modes of the operational amplifiers (Ca/Cb, and Cc), in response to current sense voltages VCSa, VCSb, and VCSc. Line waveform sensor 28 determines current-setting voltage VSET based on the present input voltage VIN, while current-setting voltage VSET substantially determines the target value of the current passing a LED in the LED string when that LED shines.
FIGS. 2A and 2B demonstrate two different luminance intensity results when LED lighting system 20 is powered by branch circuits of 200 ACV and 100 ACV, respectively, where threshold voltages VTH1, VTH2 and VTH3 are the forward voltages of the LED string with only LED 15a, the LED string with LEDs 15a and 15b, and the LED string with LEDs 15a, 15b and 15c, respectively. FIGS. 3A and 3B demonstrate the input current IIN from input voltage VIN to the LED string 14 of FIG. 1 when LED lighting system 20 is powered by branch circuits of 200 ACV and 100 ACV, respectively. Input current IIN in FIG. 3B is almost a constant when the LED string 14 is driven to illuminate. Recess 26 in FIG. 3A, which causes the happening of recess 24 in FIG. 2A, occurs, nevertheless, because there is a period of time when input voltage VIN exceeds reference voltage VIN-REF. Recess 24 helps the shadowed area in FIG. 2A to be as large as that in FIG. 2B, such that the average luminance intensity of the LED lighting system 20 could be independent to the voltage magnitude of the branch circuit.